Factorize 12x2 - 13x - 14

For $12x^2 - 13x - 14$, we have $a=12$, $b=-13$, $c=-14$.
We need two numbers that multiply to $a \times c = 12 \times (-14) = -168$ and add up to $b = -13$. These numbers are 8 and $-21$.
($8 \times -21 = -168$ and $8 + (-21) = -13$)